feat: add channel adapter abstraction with Telegram and WebChat adapters

Implement Phase 3 channel adapters that decouple message sources from
the agent via a uniform ChannelAdapter interface and ChannelRegistry.

- Add ChannelAdapter/InboundMessage/OutboundMessage types
- Add ChannelRegistry for adapter lifecycle and message routing
- Add TelegramAdapter (grammy bot, auth middleware, confirmations, chunking)
- Add WebChatAdapter (thin shim over GatewayServer)
- Refactor daemon to use ChannelRegistry with per-channel-per-user agents
- Add config.get/config.patch gateway handlers (Phase 2 loose end)
- Add system.restart gateway handler (Phase 2 loose end)
- Add implementation plans and design docs

Tests: 225 passing (33 new channel adapter + gateway handler tests)

src/gateway/handlers/system.ts

@@ -1,5 +1,5 @@
 import type { GatewayRequest, OutboundMessage } from '../protocol.js';
-import { makeResponse } from '../protocol.js';
+import { makeResponse, makeError, ErrorCode } from '../protocol.js';
 
 export interface SystemHandlerDeps {
   startTime: number;
@@ -7,6 +7,8 @@ export interface SystemHandlerDeps {
   getSessionCount: () => number;
   getToolCount: () => number;
   getConnectionCount: () => number;
+  /** Optional callback to trigger a graceful restart. If not provided, system.restart returns an error. */
+  restart?: () => Promise<void>;
 }
 
 export function createSystemHandlers(deps: SystemHandlerDeps) {
@@ -21,5 +23,23 @@ export function createSystemHandlers(deps: SystemHandlerDeps) {
         connections: deps.getConnectionCount(),
       });
     },
+
+    'system.restart': async (request: GatewayRequest): Promise<OutboundMessage> => {
+      if (!deps.restart) {
+        return makeError(request.id, ErrorCode.InternalError, 'Restart not available in this environment');
+      }
+
+      // Send response before initiating restart (client receives confirmation)
+      const response = makeResponse(request.id, { restarting: true });
+
+      // Schedule restart after response is sent (next tick)
+      queueMicrotask(() => {
+        deps.restart!().catch((err) => {
+          console.error('Restart failed:', err);
+        });
+      });
+
+      return response;
+    },
   };
 }